Areas of Syria's Hama province captured by Syrian insurgents came under heavy air attack on Thursday as government forces sought to counter a major rebel assault in an area of strategic importance to President Bashar al-Assad.

The rebel thrust in Hama marks a new challenge to Assad and his allies in a part of Syria where he has tried to consolidate his grip on power against a more than five-year-old insurgency.

The attack that began on Tuesday is the biggest coordinated rebel offensive in Hama province since 2014, according to the Syrian Observatory for Human Rights monitoring group.

The Observatory said at least 25 people including six children had been killed in the air strikes overnight.

Syrian state television said the air force had carried out "concentrated strikes" against what it described as terrorists in the area, saying tens of them had been killed.

An official in one of the rebel factions waging the attack, Jaish al-Nasr, said both Syrian and Russian jets were involved in what he described as heavy air strikes. Russia has been bombing anti-Assad forces for almost a year.

The rebel groups taking part include the jihadist Jund al-Aqsa and factions fighting under the banner of the Free Syrian Army (FSA). Jaish al-Nasr, one of the FSA groups, said in a statement released overnight that two commanders and three other of its combatants had been killed in the Hama battles.

Rebels have captured a number of towns and villages in the attack. The targeted area is populated by Christians and Alawites loyal to the government and is close to the mountain heartland of Assad's Alawite sect.

The Observatory said the air strike that killed 25 people hit a road between the town of al-Latamenah and Idlib province, an area of northwestern Syria mostly under insurgent control.

A Syrian military source said the air force had destroyed dozens of insurgent vehicles and the militants riding in them on a road from al-Latamenah to Idlib.